Glass fibers are currently used in sundry composite structural materials that are comprised of a resin matrix having fibers dispersed therein, often called Sheet Molding Composite (SMC). Examples of the large range of SMC applications include automotive, watercraft, and aircraft bodies, building structures, and containment structures. Although widely employed with glass fibers, SMC processes are not typically practiced with carbon fiber due to the insufficient wetting (wet-through, infiltration) of the carbon fiber which causes poor processability resulting in poor mechanical properties.
Multifilament carbon fiber tows (also called a bundles or ropes) comprising a plurality of carbon fiber filaments (hereinafter called filaments) are typically sold in tows, fully sized along the length thereof. Sizing is a usually proprietary coating material that is applied to an entire carbon fiber tow at 0.5 to 5 percent of the weight of the carbon fiber. A general purpose of sizing is to protect the carbon fiber during handling and processing (winding and weaving, for example) into intermediate forms, such as dry fabric and prepreg. Sizing also holds filaments together in the tow to reduce disintegration (for example, fraying, unraveling, tattering, and the like), thereby improving processability of the tow. Moreover, sizing increases interfacial shear strength between the fiber and matrix resin.
Carbon fibers have heretofore been limited in the applicability thereof to SMC materials. Current industrially adopted surface treatment processes are not sufficient to produce adequate surface functionality for desired low-cost carbon fibers that are compatible with both thermoplastic and thermoset matrix resins. For example, use of sized carbon fibers for SMC produces has been difficult because the sizing prevents sufficient wet-through of the fiber tow by the matrix resin. Thus, inadequate wet-through causes poor bonding of the matrix to the carbon fibers and significantly weakens the SMC, resulting in insufficient mechanical properties for intended applications.
Moreover, use of unsized carbon fibers is not desirable because of a condition known as fuzz and/or disintegration. The carbon fiber tows tend to fuzz and/or disintegrate when mixed with the matrix resin and become entangled and/or agglomerated, causing blockages, voids, and breakages, which significantly weaken the SMC, resulting in insufficient mechanical properties for many intended applications.
It is desirable to improve carbon fiber wettability in order to achieve a more thorough and sufficient resin wet-through into a carbon fiber tow while minimizing fuzz and disintegration.